Make transport and index versions easier to extend

When overriding transport and index versions, it is difficult to decide
whether a version constant in serverless should be returned, or the
latest version constant from serverless should be used, since the latest
from serverless is not available. This commit adjust the version
extension methods to pass in the latest serverless version constants. It
also tweaks module and method visibility for a helper method needed.

server/src/main/java/module-info.java

@@ -279,7 +279,11 @@
     exports org.elasticsearch.indices.recovery.plan;
     exports org.elasticsearch.indices.store;
     exports org.elasticsearch.ingest;
-    exports org.elasticsearch.internal to org.elasticsearch.serverless.version, org.elasticsearch.serverless.buildinfo;
+    exports org.elasticsearch.internal
+        to
+            org.elasticsearch.serverless.version,
+            org.elasticsearch.serverless.buildinfo,
+            org.elasticsearch.serverless.constants;
     exports org.elasticsearch.lucene.analysis.miscellaneous;
     exports org.elasticsearch.lucene.grouping;
     exports org.elasticsearch.lucene.queries;

server/src/main/java/org/elasticsearch/TransportVersion.java

@@ -115,7 +115,7 @@ private static TransportVersion findCurrent() {
             if (versionExtension == null) {
                 return TransportVersions.LATEST_DEFINED;
             }
-            var version = versionExtension.getCurrentTransportVersion();
+            var version = versionExtension.getCurrentTransportVersion(TransportVersions.LATEST_DEFINED);
             assert version.onOrAfter(TransportVersions.LATEST_DEFINED);
             return version;
         }

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -216,7 +216,7 @@ static TransportVersion def(int id) {
         IDS = null;
     }
 
-    static NavigableMap<Integer, TransportVersion> getAllVersionIds(Class<?> cls) {
+    public static NavigableMap<Integer, TransportVersion> getAllVersionIds(Class<?> cls) {
         Map<Integer, String> versionIdFields = new HashMap<>();
         NavigableMap<Integer, TransportVersion> builder = new TreeMap<>();
 

server/src/main/java/org/elasticsearch/index/IndexVersion.java

@@ -160,7 +160,7 @@ private static IndexVersion findCurrent() {
             if (versionExtension == null) {
                 return LATEST_DEFINED;
             }
-            var version = versionExtension.getCurrentIndexVersion();
+            var version = versionExtension.getCurrentIndexVersion(LATEST_DEFINED);
 
             assert version.onOrAfter(LATEST_DEFINED);
             assert version.luceneVersion.equals(Version.LATEST)

server/src/main/java/org/elasticsearch/internal/VersionExtension.java

@@ -18,14 +18,16 @@ public interface VersionExtension {
     /**
      * Returns the {@link TransportVersion} that Elasticsearch should use.
      * <p>
-     * This must be at least equal to the latest version found in {@link TransportVersion} V_* constants.
+     * This must be at least as high as the given fallback.
+     * @param fallback The latest transport version from server
      */
-    TransportVersion getCurrentTransportVersion();
+    TransportVersion getCurrentTransportVersion(TransportVersion fallback);
 
     /**
      * Returns the {@link IndexVersion} that Elasticsearch should use.
      * <p>
-     * This must be at least equal to the latest version found in {@link IndexVersion} V_* constants.
+     * This must be at least as high as the given fallback.
+     * @param fallback The latest index version from server
      */
-    IndexVersion getCurrentIndexVersion();
+    IndexVersion getCurrentIndexVersion(IndexVersion fallback);
 }